Microsoft suggested at the time -- as it always does -- that users wait until offered the update before installing it, but now says that it is fully available to advanced users. But what is an 'advanced user' in this context?

Rollout Status as of December 17, 2018: Windows 10, version 1809, is now fully available for advanced users who manually select "Check for updates" via Windows Update.

So, essentially, if you choose to check for an update yourself, you are viewed as an advanced user and this may have the additional side effect that preview updates -- that is updates that are still in testing -- might also be installed. This is something Microsoft revealed here.

So should you install the October 2018 Update yourself? Probably not. The update will roll out to devices automatically in due course, so you might as well wait until offered it, especially if you don’t want to risk being served pre-release software at the same time.

If your device is currently one of those blocked from receiving the October 2018 Update due to compatibility issues, that wait will go on anyway, as you won’t be able to download the new version via Windows Update.